Most Webmasters and Google AdSense publishers know that the AdSense code you place on your site includes your unique AdSense publisher ID. So if you ever wanted to track down all the places and sites that AdSense ID is used on the web, technically you can just search for the unique ID on various web sites.
Well, Blekko has made it easier.
If you type in any site, such as this or even [TechCrunch] into Blekko, you will see a link for "adsense" under the title of the page. Clicking on that will take you to a search for [/adsense=6181816114362650] which is the AdSense ID used on TechCrunch. It then lists out all the sites on the web using that ID.
Now, keep in mind, a couple things:
(1) Scrapers will often copy your AdSense code and AdSense ID and include it on their sites.
(2) Your competitors may have additional AdSense accounts, all with unique IDs.
Forum discussion at WebmasterWorld.